This is a tricky question.
Right now, the most common Li-ion rechargeable is the 18650. As soon as Tesla ramps up battery production for the Tesla Model 3, however, that will change. Tesla has announced that the Model 3 will use 21-70 (a.k.a 21700) batteries. They are 21mm in diameter and 70mm in length. The large number that Tesla will need means that the 21-70 format will immediately become the most common Li-ion battery.
Many members here speculate that 21-70 will become available, and perhaps common, in flashlight designs within a few years. Whether it will replace 18650 in the long run has yet to be seen.
For those who have seen CD, DVD, and even Blu-ray, fade in popularity, it is hard to predict that Li-ion formats will remain unchanged. The best we can do is try to look forward, say, five or more years, and make our best guess. 18650 is almost certain to be viable over that time range. Ten or twenty years downstream is anybody's guess.
If you want to look forward twenty years, your best bet, by far, is AA. That's a format that has been around for more than a century.
Is 21-70 the future? What do you think?